1. Field of the Invention
The present invention relates to a semiconductor device having a plurality of types of transistors respectively having different threshold voltages and a method of manufacturing the same.
2. Description of the Related Art
At present, although the threshold voltage of a normal transistor is set to be 0.5 to 0.6 V, when a power supply voltage for a transistor decreases, or the high-speed operation of the transistor is demanded, the threshold voltage must be decreased. However, when the threshold voltages of all transistors are uniformly decreased, the sub-threshold current of the transistors or the stand-by current of the circuit constituted by these transistors increases, and an operating current also increases, thereby posing a problem such as an increase in power consumption.
For this reason, it is considered that an operation at a low voltage can be performed by decreasing the threshold voltages of only the transistors of a CMOS analog switch, an oscillation circuit, a DRAM or SRAM sense amplifier and the like, which transistors do not increase the stand-by current of the circuit even when the threshold voltages of these transistors are decreased. On the other hand, in an access transistor constituting a DRAM memory cell, when a threshold voltage is decreased, a sub-threshold current increases, thereby degrading data retention characteristics. Therefore, the threshold voltage is preferably set to be a relatively high voltage.
For this reason, according to a conventional technique, a resist pattern having an opening on only a transistor whose threshold voltage is to be decreased or a resist pattern having an opening on only a transistor whose threshold voltage is to be increased is additionally formed, and ion implantation for adjusting an impurity concentration on the surfaces of the channel portions of these transistors is performed using these resist patterns as masks.
However, according to the above conventional method, since an additional resist is used to adjust a threshold voltage, the step of forming the additional resist is required. Therefore, the manufacturing cost of a semiconductor device increases.